Hún spilar á píanó í stofunni á kvöldin.

Spilar is the 3rd person singular present tense of the verb spila.

So:

• spila = to play
• hún spilar = she plays

It matches the subject hún because hún means she.

With musical instruments, Icelandic very often uses the pattern:

• spila á + instrument

So:

• spila á píanó = play the piano
• spila á gítar = play the guitar

This is just the normal Icelandic construction. English uses play the piano, but Icelandic uses a preposition here.

Also, Icelandic often leaves out the article in this kind of expression, so á píanó sounds natural even though English normally says the piano.

The two á's do different jobs:

• á píanó = part of the expression spila á píanó = play the piano
• á kvöldin = in the evenings

This is very common in Icelandic: the same preposition can have different meanings in different expressions. You should not expect one Icelandic preposition to match one English preposition perfectly.

Because í here shows location: she is playing in the living room, not moving into it.

When í means in in the sense of being located somewhere, it takes the dative. The noun stofa therefore changes form:

• stofa = living room
• stofunni = in the living room

The ending also includes the definite article, so stofunni means the living room, not just a living room.

It is doing two things at once:

1. showing the dative singular
2. adding the definite article (the)

So stofunni means something like:

• stofa = living room
• stofan = the living room
• í stofunni = in the living room

This is very typical in Icelandic, where the definite article is usually attached to the end of the noun instead of being a separate word.

Because it means something habitual or repeated: in the evenings.

Compare the idea in English:

• in the evening = one evening / one part of the day
• in the evenings = regularly, evening after evening

So á kvöldin tells you this is something she does as a usual or repeated activity.

Yes, kvöldin is the definite plural form, literally something like the evenings.

In Icelandic, expressions for repeated times often use this kind of form:

• á morgnana = in the mornings
• á kvöldin = in the evenings

Even though English often does not feel very definite in these expressions, Icelandic commonly uses the definite plural form anyway. So it is best to learn á kvöldin as a standard time expression.

Yes. Icelandic word order is fairly flexible, especially with time and place phrases.

This sentence is perfectly normal:

• Hún spilar á píanó í stofunni á kvöldin.

But you could also move other elements to the front for emphasis, for example:

• Á kvöldin spilar hún á píanó í stofunni.
• Í stofunni spilar hún á píanó á kvöldin.

Notice something important: if another element comes first, the finite verb usually stays in the second position. That is a key pattern in Icelandic.

Normally, yes, hún should be included if you want to say she clearly.

Icelandic usually keeps subject pronouns more than some other languages do. The verb ending by itself often is not enough to leave the subject out naturally in an ordinary sentence.

So:

• Hún spilar ... = normal, clear
• Spilar ... by itself would usually sound incomplete unless the context already strongly supplies the subject, or it is a question.

Spila can be used for several kinds of play, including music and games, depending on the context.

In this sentence, the phrase spilar á píanó clearly shows that it means plays the piano.

So the context tells you what kind of play is meant:

• spila á píanó = play the piano
• spila leik / spila fótbolta / spila spil = other kinds of playing, depending on the words used

The prepositional phrase á píanó makes the musical meaning unmistakable.